Papermaking processes require treatment of a system comprising a liquid dispersion of solid particles for separating the solids therefrom. Fast drainage and greater retention of fines contribute to lower costs in papermaking and thus improvements in this ares are always being sought. Improvements in formation are likewise desired as such improvements result in a better product. One method for improving these properties, which was first practiced during the 1980's, involves the use of colloidal silica and bentonite. The improved drainage offered with the use of these materials, i.e., as indicated by increasing speed and efficiency with greater retention of fines, provides significant cost savings over the prior art techniques.
U.S. Pat. Nos. 4,385,165 and 4,388,150 describe a two-component binder system comprising a cationic starch and an anionic, colloidal silicic acid sol which acts as a retention aid when combined with cellulose fibers in a paper-making stock. Finnish published specification Nos. 67,735 and 67,736 disclose cationic polymer retention agent compounds comprising cationic starch and polyacrylamide. These materials are described by the subject references as being useful when combined with an anionic silica in improving sizing.
U.S. Pat. No. 4,798,653 discloses the use of cationic colloidal silica sol in combination with an anionic copolymer of acrylic acid and acrylamide for rendering paper stock resistant to loss of its retention and dewatering properties due to shear forces attributable to the papermaking process.
A coacervate binder, three-component system composed of a cationic starch, an anionic high molecular weight polymer and dispersed silica having a particle diameter range from 1 to 50 nm is described in U.S. Pat. Nos. 4,643,801 and 4,750,974.
The two Finnish patent publications noted above additionally describe the use of bentonite with cationic starch and polyacrylamides ("PAMs"). Further, U.S. Pat. No. 4,305,781 discloses a bentonite-type clay used in combination with high-molecular weight, substantially non-ionic polymers such as polyethylene oxides and PAMs for use as retention agents. U.S. Pat. No. 4,753,710 discloses the use of bentonite with a substantially linear, cationic polymer, e.g., cationic acrylic polymers, polyethylene imine, polyamine epichlorohydrin and dialkyl dimethyl ammonium chloride as providing an improved combination of retention, drainage, drying and formation.
Another material which has been found useful in separating particulate dispersions of the type contemplated herein is organic crosslinked microbeads. Such microbeads are known to be particularly useful for flocculating a wide variety of dispersions of suspended solids as described for example in U.S. Pat. No. 5,171,808.
The use of such organic crosslinked microbeads in papermaking is taught, e.g., in U.S. Pat. No. 5,180,473. The '473 reference discloses a dual system comprising a cationic organic microbead of 1-100 microns together with an anionic, cationic or nonionic acrylamide polymer. The cationic polymer particle is of the water swelling type and is a crosslinked homopolymer of 2-methacryloyloxyethyl trimethylammonium chloride or a crosslinked copolymer of 2-methacryloxy-ethyl trimethylammonium chloride/acrylamide (60/40 weight percent). The acrylamide polymer is an acrylamide homopolymer or acrylamide hydrolysate of 17 mole percent anion-conversion or a copolymer of acrylamide/2-methacryloyloxyethyltrimethyl ammonium chloride (75/25 weight percent). Japanese Patent Publication No. JP 235596/63:1988, which corresponds to the U.S. '473 patent, discloses the use of both cationic and anionic microbeads. The anionic microbead disclosed by the Japanese reference is an acrylamide-acrylic acid copolymer.
European Patent No. 0 202 780 describes the preparation of cross-linked cationic polyacrylamide beads by conventional inverse emulsion polymerization techniques. During formation of the beads, the PAM is crosslinked by incorporating a difunctional monomer, such as methylene bis-acrylamide, in a manner well known in the art into the polymer chain. The reference further discloses that the cross-linked beads, while useful as flocculants, are more highly efficient after having been subjected to unusual levels of shearing action in order to render them water soluble.
Typically, the particle size of polymers prepared by conventional, inverse, water-in-oil emulsion polymerization processes is limited to the 1-5 micron range since there is no particular advantage known to reduce this particle size. The particle size achievable in inverse emulsions is determinable by the concentration and activity of the surfactants employed, which surfactants are customarily chosen based on the desired emulsion stability as well as on economic factors.
U.S. Pat. No. 5,167,766 discloses the addition, in a papermaking process, of ionic, organic microbeads of up to about 750 nm in diameter to obtain improved drainage, retention and formation. These microbeads may be made as microemulsions, or as microgels, or they may be obtained commercially as microlatices. The microbeads may be added either alone or in combination with a high molecular weight polymer and/or a polysaccharide. Other standard paper-making additives, including particularly alum or any other active, soluble aluminum species, also may be added for their well known purposes.
In view of the importance to, for example, the papermaking industry, of improving drainage, retention and formation during the separation of solid particles from liquid particulate dispersions, those working in this field are constantly on the lookout for compositions and methods which are particularly efficient in improving these properties.